The present invention relates to lubricating oil compositions. More specifically, the present invention relates to lubricating oil compositions which are extremely excellent in oxidation stability, base number retention properties, anti-wear properties, extreme pressure properties and anti-corrosion properties, particularly suitable for internal combustion engines.
Conventionally, lubricating oils have been used in internal combustion engines and automatic transmissions so as to facilitate the smooth operation thereof. In particular, lubricating oils for internal combustion engines (engine oils) have been required to possess high characteristic performances due to the fact that recent developed engines have been improved in performances, increased in power output and used under more severe operating conditions and further required to be improved in long-drain properties enabling the prolongation of oil drain intervals from the view point of recent environmental issues. Therefore, in order to fulfill such requirements, conventional engine oils are blended with various additives such as anti-wear agents, metallic detergents, ashless dispersants, and anti-oxidants so as to improve their characteristic performances.
The inventors of the present invention have already found that a lubricating oil composition containing less or no zinc dithiophosphate that is an anti-wear and anti-oxidation agent having been used conventionally as an indispensable additive for internal combustion engine lubricating oils, but containing a specific phosphorus-containing compound was able to exhibit extremely excellent long-drain properties (oxidation stability, base number retention properties and thermal stability) while maintaining anti-wear properties equally to a composition containing zinc dithiophosphate. As the result, the inventors filed patent application for such a lubricating oil composition, as disclosed in Japanese Patent Laid-Open Publication Nos. 2002-294271 and 2004-83751.
It was confirmed that a lubricating oil composition containing such a specific phosphorus compound when optically blended with other additives was able to exhibit anti-wear properties evaluated by a valve train wear test for domestically produced automobile engines as represented by JASO M328-95 comparably to a composition containing zinc dithiophosphate. However, the lubricating oil is required to possess extreme pressure properties and anti-wear properties more excellent than ever so as to be used in a special engine operating under more severe conditions or used under particular circumstances where more excellent extreme pressure properties and anti-wear properties are required; or required to fulfill a requirement that the phosphorus content is decreased to 0.08 percent by mass or less to meet the suitableness for an exhaust-gas purifying catalyst in the forthcoming ILSAC GF-4 standard or another requirement of low phosphorus content that the phosphorus content is decreased to 0.05 percent by mass or less to be sought in ILSAC GF-5 standard which is a plan under consideration. However, many of the sulfur-containing extreme pressure additives not only extremely degrade base number retention properties but also exert harmful influence on an exhaust-gas after-treatment device, i.e., fails to solve the problems that an exhaust-gas purifying catalyst such as a three-way catalyst, an oxidation catalyst and a NOx adsorber and DPF or an exhaust-gas treatment system which is the combination of DPF with the exhaust-gas purifying catalyst, in particular the oxidation catalyst or NOx adsorber undergo to catalyst poisoning and/or clogging of DPF caused by the increased sulfur. Therefore, it has been very difficult to produce a lubricating oil which can achieve the decrease of phosphorus and sulfur or additionally ash content while maintaining excellent long-drain properties, extreme pressure properties and anti-wear properties and reducing adverse affect on the exhaust-gas after-treatment device.